This invention relates to apparatus for heat sealing plastic sheets and, more particularly, heat sealing apparatus having rapid cooling means.
In the process of impulse heat sealing of thermoplastic materials, an electrically conductive wire is generally heated by a short burst of electrical current. Typically, it is necessary to continue maintaining the wire and the plastic under pressure until the plastic is sufficiently cooled, otherwise the heat seal may be defective.
Using ambient cooling, an eight-second cooling period is common. If water cooling is used, the cooling time may be reduced. However, water cooling may be a problem because it requires the constant circulation of water. Since the wire heating element must be replaced frequently, such replacement is difficult where water cooling is involved because the water-carrying assembly must be disassembled. Thus water cooling is a relatively messy operation. Additionally, water cooling is relatively inefficient because while the water aids to cool the heat seal head, the wire heating element is only cooled indirectly.
Atomized liquid cooling of the wire heating element in the impulse heat sealing of thermoplastic film has been disclosed in the prior art. For example, such atomized liquid cooling is disclosed in Anderson U.S. Pat. No. 3,496,049. However, the Anderson device is disadvantageous in that Anderson does not disclose any coolant fluid exhaust and, once the top jaw of Anderson's machine is closed, circulation of Anderson's cooling fluid may be inadequate.
Many of the disadvantages of prior art heat sealing apparatus are alleviated by the present invention. It is an object of the present invention to provide heat sealing apparatus with cooling means whereby the cooling means is effectively circulated for rapid cooling of the wire heating element.
Another object of the present invention is to provide heat sealing apparatus in which the cooling fluid is in direct contact with the wire heating element.
A further object of the present invention is to provide heat sealing apparatus in which the wire heating element is longitudinally cooled by overlying a number of longitudinal slots to which a cooling fluid is passed.
A still further object of the present invention is to provide heat sealing apparatus which is simple in construction and operates to significantly reduce the time for cooling down the wire heating element, thereby enabling more rapid heat sealing of thermoplastic materials.